1. Field of the Invention
This invention relates to a wavelength characteristic measurement apparatus for measuring the light wavelength characteristic of an optical component such as an optical filter or an optical transmission line and more particularly to a wavelength tracking control technique in wavelength characteristic measurement using an optical spectrum analyzer for measuring the spectrum of an optical signal and a wavelength variable light source capable of outputting different wavelengths.
2. Description of the Related Art
FIG. 21 is a block diagram to show an apparatus configuration example in a related art for embodying wavelength tracking in wavelength characteristic measurement using an optical spectrum analyzer and a wavelength variable light source. In the figure, numeral 100 denotes an optical spectrum analyzer for measuring an optical spectrum and numeral 101 denotes a wavelength variable light source capable of outputting different wavelengths.
The optical spectrum analyzer 100 comprises a control section 102 for controlling the whole operation of the optical spectrum analyzer 100, a communication circuit 103 for carrying out communications with an external machine (in this case, the wavelength variable light source 101), a terminal {circle around (3)} used as an input/output interface of the communication circuit 103, a spectroscope 104 for extracting and outputting a specific wavelength from measured light by a spectrum using a spectral element of a diffraction grating, a prism, an interference filter, etc., an optical input terminal 105 for inputting measured light given from the outside to the spectroscope 104, a motor 106 for varying the extracted wavelength of the spectroscope 104, a drive circuit 107 for driving the motor 106 in accordance with the conditions of the motor rotation speed, rotation quantity, etc., set from the control section 102, a position detection circuit 108 for detecting the rotation quantity and the rotation position of the motor 106, a photodetector 110 for receiving the extracted light output from the spectroscope 104 and converting the light into an electric signal, an amplification circuit 111 for amplifying the minute electric signal output from the photodetector 110, an A/D (analog-digital) converter 112 for quantizing an analog signal output from the amplification circuit 111 and converting the signal into a digital signal, and a display section 113 for displaying an optical spectrum provided by plotting measurement data output from the A/D converter 112.
The control section 102 drives the motor 106 by the drive circuit 107 to set the extracted wavelength of the spectroscope 104 to any desired value based on motor control information previously stored in the control section 102. The control section 102 checks position information provided by the position detection circuit 108 connected to the motor 106 to ensure that the extracted wavelength of the spectroscope 104 is set to the desired value, then reads measurement data from the A/D converter 112, performs predetermined operation processing, and displays the operation result on the display section 113.
At this time, the control section 102 finds each wavelength at equal intervals as the extracted wavelength of the spectroscope 104 based on the wavelength range and the number of measurement samples set by the measurer and while intermittently finding measurement data provided when the extracted wavelength of the spectroscope 104 is set to each wavelength at equal intervals, the control section 102 plots the measurement data on the display section 113, whereby it is made possible to display the measurement waveform of the spectrum concerning the measured light.
The optical spectrum analyzer 100 has a function as a host for controlling the external wavelength variable light source 101 connected Lo the optical spectrum analyzer 100. That is, the control section 102 transmits a control instruction to a control section 115 (described later) of the wavelength variable light source 101 via the communication circuit 103 and the terminal {circle around (3)} and a terminal {circle around (3)}xe2x80x2 and a communication circuit 116 (both described later) of the wavelength variable light source 101, thereby setting the wavelength and light power of signal light output by the wavelength variable light source 101.
A measured object is an optical component whose wavelength characteristic such as a wavelength versus loss characteristic is to be measured, such as an optical component for WDM (wavelength division multiplexing), an optical fiber grating, or a dielectric multilayer film filter. Measured light is supplied from the measured object 114 to the optical spectrum analyzer 100 by wavelength tracking measurement using single mode signal light supplied from an optical output terminal 118 (described later) of the wavelength variable light source 101, and the wavelength characteristic of the measured object 114 is measured.
On the other hand, the wavelength variable light source 101 comprises a control section for controlling the whole operation of the wavelength variable light source 101, the communication circuit 116 for carrying out communications with an external machine (in this case, the optical spectrum analyzer 100), the terminal {circle around (3)}xe2x80x2 used as an input/output interface of the communication circuit 116, the optical output terminal 118 for outputting an optical signal output from a light source 122 (described later) to the external measured object 114, the light source 122 for oscillating a single mode spectrum with its oscillation wavelength being variable, a light source drive circuit 123 for driving the light source 122 and performing temperature control, etc., of the light source 122, a display section 124 for displaying conditions of the measurement wavelength range, etc., set by the measurer (described later in detail), and a wavelength control circuit 126 for controlling the wavelength of the optical signal output from the light source 122.
The control section 115 controls the light source drive circuit 123 and the wavelength control circuit 126 based on the light source drive information and the wavelength information previously stored in the control section 115 and varies single mode oscillation wavelength and oscillation light power of the light source 122. That is, the control section 115 finds parameters of the measurement wavelength interval, etc., (described later in detail) based on the setup conditions by performing operations, gives instructions to the light source drive circuit 123 and the wavelength control circuit 126, and oscillates the light source 122 under an arbitrary setup condition. In addition, the control section 115 intermittently changes the oscillation wavelength of the light source 122 at predetermined wavelength intervals throughout the wavelength range set by the measurer. Like the optical spectrum analyzer 100, the wavelength variable light source 101 also uses a spectral element (not shown) and a motor (not shown) for driving the spectral element to vary the oscillation wavelength of the light source 122.
Next, the procedure of wavelength tracking control performed in the wavelength characteristic measurement apparatus in the related art will be discussed according to a flowchart shown in FIG. 22. The control in the optical spectrum analyzer 100 and the wavelength variable light source 101 as described below may be performed using a computer, etc., provided aside from them. First, the measurer sets measurement conditions of measurement start wavelength xcex0, measurement end wavelength xcexe, the number of measurement samples, etc., in the optical spectrum analyzer 100. Then, the control section 102 derives parameters of wavelength interval xcex94xcex, etc., by performing operations based on the setup measurement conditions (step S1).
Next, the control section 102 sends a signal to the drive a circuit 107 in accordance with the found parameters, thereby driving the motor 106 for setting the extracted wavelength of the spectroscope 104 to one initial wavelength, and checks position information output from the position detection circuit 108 to ensure that the extracted wavelength of the spectroscope 104 is set to the initial wavelength. The control section 102 also transmits a xe2x80x9cmove command to initial wavelengthxe2x80x9d to the wavelength variable light source 101 via the communication circuit 103 and the terminal {circle around (3)} (step S2).
Then, the control section 115 of the wavelength variable light source 101 finds parameters given to the light source drive circuit 123 and the wavelength control circuit 126 by performing operations based on the command transmitted via the communication circuit 116 from the optical spectrum analyzer 100, and supplies the parameters to the circuits. The oscillation wavelength of the light source 122 is set to the initial wavelength based on the parameters. After the termination of the setting, a xe2x80x9cwavelength setting completion commandxe2x80x9d is transmitted to the control section of the optical spectrum analyzer 100 via the communication circuit 116 (if the decision result at step S3 is YES).
Next, when the measurer gives a measurement start instruction to the optical spectrum analyzer 100 (step S4), the control section 102 sends a control signal to the drive circuit 107 for driving the motor 106 so that the extracted wavelength xcex of the spectroscope 104 becomes the measurement start wavelength xcex0, monitors position information output from the position detection circuit 108, and waits for move completion of the motor 106. The control section 102 transmits a xe2x80x9cmove command to measurement wavelength xcex (namely, measurement start wavelength xcex0)xe2x80x9d to the wavelength variable light source 101 via the communication circuit 103, then waits for return of a xe2x80x9cwavelength setting completion commandxe2x80x9d from the wavelength variable light source 101 as with the case of the initial wavelength described above (step S5).
If wavelength setting in the optical spectrum analyzer 100 and the wavelength variable light source 101 is thus complete (if the decision result at step S6 is YES), the control section 102 starts the A/D converter 112 and reads a digital signal from the A/D converter 112 (step S7), then calculates the light power value based on the preset condition of the amplification circuit 111 and the like and plots the light power value on the display section 113 (step S8).
Next, the control section 102 finds the next measurement wavelength xcex at a distance of wavelength interval xcex94xcex from the current measurement wavelength xcex it by performing operation (step S9) and again sets the extracted wavelength of the spectroscope 104 and the output wavelength of the wavelength variable light source 101. Then, the operation at steps S5 to S9 is repeated in the optical spectrum analyzer 100 and the wavelength variable light source 101. The control section 102 continues the wavelength setting and data measurement until the measurement wavelength exceeds the measurement end wavelength xcexe (the decision result at step S10 is YES).
Thus, in the wavelength tracking control in the related art in the optical spectrum analyzer and the wavelength variable light source, the optical spectrum analyzer 100 as the host controls the operation of the wavelength variable light source 101 via the communication interface. Then, the extracted wavelength of the spectroscope 104 and the output wavelength of the wavelength variable light source 101 are intermittently set for measurement every measurement wavelength interval found over the setup sweep wavelength range (measurement wavelength range) and further the commands are transferred between the optical spectrum analyzer 100 and the wavelength variable light source 101 via the communication interface. Thus, a problem of requiring much time for the wavelength tracking is involved.
To sweep the spectroscope 104 of the optical spectrum analyzer 100 and the light source 122 of the wavelength variable light source 101, the change characteristic of the extracted wavelength relative to the rotation quantity of the motor 106 for driving the spectroscope 104 rarely matches the change characteristic of the output wavelength relative to the rotation quantity of the motor for driving the spectral element of the wavelength variable light source 101; the change characteristics differ from each other in most cases. The reason why they differ is that the change characteristic of the spectroscope 104 is determined by various factors of the characteristic of the spectral element forming the spectroscope 104, the technique for varying the angle of the spectral element, the layout of the spectroscope 104, etc., and that the change characteristic concerning the spectral element of the wavelength variable light source 101 is also determined by various similar factors.
Thus, if the motors installed in the optical spectrum analyzer 100 and the wavelength variable light source 101 are rotated at uniform rate, the extracted wavelength of the spectroscope 104 and the output wavelength of the wavelength variable light source 101 relative to the rotation quantities of the motors change in response to their respective characteristics. Thus, the wavelength difference between the extracted wavelength and the output wavelength becomes large and it becomes impossible to perform tracking; consequently, wavelength tracking can be executed only in a very small wavelength range.
It is therefore an object of the invention to provide a wavelength characteristic measurement apparatus wherein to execute wavelength characteristic measurement using an optical spectrum analyzer and a wavelength variable light source, both sweeps are synchronized with each other, the wavelength characteristic measurement apparatus capable of matching the extracted wavelength of the optical spectrum analyzer and the output signal light wavelength of the wavelength variable light source with each other or making a narrow wavelength difference therebetween and performing wavelength tracking at high speed and with high wavelength accuracy over a wide wavelength range.
To the end, according to a first aspect of invention, there is provided a wavelength characteristic measurement apparatus using a wavelength variable light source for outputting signal light and an optical spectrum analyzer for measuring a spectral distribution of measured light provided by making the signal light incident on a measured object to measure the optical wavelength characteristic of the measured object, characterized in that the optical spectrum analyzer comprises spectral means for extracting a specific wavelength component from the measured light by a first spectral element and first drive means for varying the angle of the first spectral element and sweeping the extracted wavelength over a predetermined sweep wavelength range, that the wavelength variable light source comprises an external oscillator made up of a laser element for outputting single mode signal light as the signal light and a second spectral element for causing the laser element to lase at an arbitrary wavelength and second drive means for varying the angle of the second spectral element and sweeping the signal light wavelength of the signal light over the sweep wavelength range, and that the first and second drive means have each means for synchronizing sweep start with each other and vary and control the rotation angle of the first or second spectral element in accordance with a rotation correction function derived from the extracted wavelength characteristic relative to the rotation angle of the first spectral element and the signal light wavelength characteristic relative to the rotation angle of the second spectral element so that the extracted wavelength and the signal light wavelength match over the sweep wavelength range.
According to a second aspect of the present invention, in the wavelength characteristic measurement apparatus in the first aspect, the first drive means has a first motor for driving the first spectral element and controlling the rotation quantity and rotation speed of the first motor, thereby matching the extracted wavelength characteristic with the signal light wavelength characteristic, so that the extracted wavelength and the signal light wavelength are matched with each other over the sweep wavelength range.
According to a third aspect of the present invention, in the wavelength characteristic measurement apparatus in the first aspect, the second drive means has a second motor for driving the second spectral element and controlling the rotation quantity and rotation speed of the second motor, thereby matching the extracted wavelength characteristic with the signal light wavelength characteristic, so that the extracted wavelength and the signal light wavelength are matched with each other over the sweep wavelength range.
According to a fourth aspect of the present invention, there is provided a wavelength characteristic measurement apparatus using a wavelength variable light source for outputting signal light and an optical spectrum analyzer for measuring a spectral distribution of measured light provided by making the signal light incident on a measured object to measure an optical wavelength characteristic of the measured object, characterized in that the optical spectrum analyzer comprises spectral means for extracting a specific wavelength component from the measured light by a first spectral element, the spectral means having a wavelength transmission characteristic wherein the maximum transmission quantities are flat over a predetermined wavelength range preceding and following the extracted wavelength as the center relative to a preset resolution, and first drive means for varying the angle of the first spectral element and sweeping the extracted wavelength over a predetermined sweep wavelength range, that the wavelength variable light source comprises an external oscillator made up of a laser element for outputting single mode signal light as the signal light and a second spectral element for causing the laser element to lase at an arbitrary wavelength and second drive means for varying the angle of the second spectral element and sweeping the signal light wavelength of the signal light over the sweep wavelength range, and that the rotation speeds of the first and second motors for varying the angles of the first and second spectral elements respectively are previously determined so that the signal light wavelength converses in the predetermined wavelength range and the first and second drive means rotate the first and second motors at uniform rate in accordance with the rotation speeds.
According to a fifth aspect of the present invention, in the wavelength characteristic measurement apparatus in the fourth aspect, the sweep wavelength range is divided into a plurality of wavelength sections each wherein the signal light wavelength converses in the predetermined wavelength range, and the first and second drive means rotate the first and second motors at uniform rate in accordance with the rotation speeds of the first and second motors previously determined for each of the wavelength sections.
According to a sixth aspect of the present invention, in the wavelength characteristic measurement apparatus in the fifth aspect, the first and second drive means vary the pulse rates of the first and second motors in the middle of outputting a motor rotation pulse to the first and second motors.
According to a seventh aspect of the present invention, in the wavelength characteristic measurement apparatus in the fifth aspect, the first and second drive means once stop the sweep operation for each of the wavelength sections and set conditions concerning the next wavelength section to be swept, then sweep this wavelength section.